This invention relates generally to semiconductor integrated circuits having low voltage detect and power-on reset, and, more particularly, to an integrated circuit using a bandgap voltage comparator having a low voltage threshold with long term voltage stability over a wide temperature range.
Electronic devices, such as, cellular telephones, laptop computers, keyless and wireless entry devices, and other integrated circuit based products may be powered by, for example, a battery, a fuel cell, a solar cell, a generator and the like require a stable and accurate voltage reference for effective and predictable operation. It is important that an accurate and reliable way of initiating the start-up and shut-down of electronic circuits in these battery powered devices over all possible operating conditions be used. During start-up (power-up) a minimum voltage value must be reached before beginning operation thereof, and during operation whenever a voltage goes below a critical value the device must be stopped or inhibited from further operation. Typical applications that perform these functions are power-on reset (POR) and power-low reset (or brown-out reset (BOR)).
POR and BOR circuits typically use a precision voltage reference in combination with a voltage comparator circuit(s) for determining if the critical voltage value has been reached at which a device may properly operate. Typical voltage references used in integrated circuits have been buried Zener and bandgap references. The buried Zener is a very stable and accurate voltage reference, however, it typically operates at about 5 volts or more and draws several hundred microamperes for optimum operation. The newer battery powered electronic systems may run at a battery voltage of 2 volts or less, thus, the buried Zener technique is not suitable as a voltage reference which must run from such a low voltage and also have low power consumption. For such applications a xe2x80x9cbandgap referencexe2x80x9d may be utilized.
A bandgap voltage reference, typically 1.2 volts, may be generated with a semiconductor circuit. This reference may be hereinafter referred to as a bandgap voltage and may be compensated for variations over changes in temperature by combining a negative temperature coefficient voltage circuit with a positive temperature coefficient voltage circuit to produce a substantially zero temperature coefficient voltage circuit, i.e., the bandgap voltage value remains substantially the same over a wide temperature range. A precision bandgap voltage reference circuit is more fully described in U.S. Pat. No. 5,900,773 by David M. Susak, and is incorporated by reference herein for all purposes.
A bandgap reference in combination with a voltage comparator may be used to sense operating voltage levels for POR and BOR circuits. The bandgap reference and comparator may be combined into one circuit such as disclosed in U.S. Pat. No. 5,781,043 by Willam Slemmer, and entitled xe2x80x9cDIRECT CURRENT SUM BANDGAP VOLTAGE COMPARATORxe2x80x9d (referred to hereinafter as xe2x80x9cSlemmerxe2x80x9d). The Slemmer patent discloses a direct current sum bandgap voltage comparator for detecting voltage changes in a power supply. Upon detecting a power supply voltage level below a certain value, the Slemmer comparator will cause a transfer switch to change the power source to a backup battery. The Slemmer bandgap voltage comparator uses four current sources summed together to produce a summing node voltage level, and generates a logic signal that indicates when the summing node voltage is greater than or equal to, or less than a predetermined value. The predetermined value corresponds to a desired power supply voltage switchover value.
The prior art voltage references and comparator circuits are too complex, draw too much current, require operating voltage levels higher than are available in the newer battery operated electronic systems, and suffer from temperature and voltage stability variations. Thus, a need exists to provide an improved voltage comparator having low operating current, a stable voltage reference over a wide range of operating temperatures, and simple and reliable implementation in an integrated circuit.
The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing an improved bandgap voltage comparator which may be utilized with other circuits in integrated circuit devices as a low voltage detection circuit. The integrated circuit devices may be, for example but not limited to, the complementary metal oxide semiconductor (CMOS) PIC Microcontroller family manufactured by Microchip Technology Inc., more fully described at http://www.microchip.com/ and incorporated by reference herein for all purposes. The present invention may operate at one volt and above, and only draw a relatively small amount of current in operation. Implementation of the embodiments of the invention in an integrated circuit is efficient and only requires a small area of the die. Accurate tip points are preferably plus or minus 50 millivolts over all process comers and industrial temperature ranges. In embodiments of the invention, power up and power down trip points can be set independently, i.e., programmable hysteresis. These trip points are insensitive to VDD rise and fall times. Embodiments of the invention may have immediate POR or BOR generation on a low voltage condition, i.e., falling VDD. A hold-off time, for example but not limitation, may be 1-10 microseconds (programmable) for a POR signal after detection of a rising trip point. The embodiments of the invention may draw no current when placed in a standby mode (sleep mode), and can evaluate voltage levels when taken out of the standby mode. A low voltage condition signal will be generated if a low voltage condition exists upon activation from the standby mode. The present invention is thus well suited for both power-on reset (POR) and power-low reset (brown-out reset BOR) modes. The predictable and efficient operation of this circuit will ensure reliable operation for all battery (2 volts or more) operated applications.
In accordance with the embodiments of the present invention, an improved bandgap voltage comparator circuit may be implemented on a semiconductor integrated circuit substrate using standard fabrication processes. Standard PNP transistors such as, for example but not limitation, lateral PNP (LPNP) transistors may be used in a circuit needing only two current mirrors to form a bandgap voltage comparator. Using only two current sources greatly simplifies the bandgap circuit and improves trip voltage stability with changes in temperature. The areas of the PNP transistors used for the second current mirror circuit may be from about four to forty eight times the area of the PNP transistors used for the first current mirror circuit, depending on the gain desired for the comparator circuit. An exemplary schematic diagram of the basic bandgap comparator of the present invention is illustrated in FIG. 4A and is described in more detail hereinbelow.
A bandgap comparator circuit, according to the embodiments of the invention, having an adjustable trip point is illustrated in the schematic diagram of FIG. 4 and is described in more detail hereinbelow. A graphical representation of an operational simulation of the embodiment of the bandgap comparator low voltage detection system of FIG. 2 is illustrated in FIG. 11. An output buffer circuit that may be used in combination with the bandgap comparator circuit of FIG. 4 is illustrated in FIG. 5. A power-on reset delay circuit that may be used with the bandgap comparator of FIG. 4 is illustrated in FIG. 6 and is described in more detail hereinbelow. An output latch with power down circuit that also may be used with the bandgap comparator circuit of FIG. 4 is illustrated in FIG. 7 and is described in more detail hereinbelow.
Bandgap voltage operation in integrated circuit devices is more fully described in xe2x80x9cAnalysis and Design of Analog Integrated Circuits,xe2x80x9d by Paul Gray and Robert Meyer, (1984, second edition), John Wiley and Sons, ISBN 0-471-897493-0, and is incorporated by reference herein for all purposes.
A feature of the present invention is operation at one volt and above by a bandgap voltage comparator.
Another feature is low current operation.
Another feature is that power-up and power-down trip points can be set independently.
Another feature is an immediate power-on reset (POR) signal generated upon detection of a low voltage supply condition.
Another feature is time delayed hold-off of a POR signal OK after detection of a rising voltage supply going above a desired voltage value (trip point).
Another feature is substantially no power being used when in a standby mode.
Another feature is evaluating voltage level when taken from a standby to an active mode and generating a POR signal if a low voltage condition exists.
An advantage of the present invention is that it may be implemented on an integrated circuit using standard semiconductor fabrication processes while only requiring a small area for fabrication thereon.
Another advantage is low current operation.
Another advantage is trip points are insensitive to voltage rise and fall times.
Another advantage is accurate trip points, preferably, plus or minus about 50 millivolts over all process comers and industrial temperature ranges.
The foregoing and other features and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.